The basic weld cycle necessary for sterily welding round, dry tubing is known in the art and is currently being practiced in the form of the DuPont SCD-I (U.S. Pat. No. 4,369,779). In attempting to extend the technology to fluid-filled tubes that welder was modified primarily by the incorporation of a set of tube clamps which flatten the tube in the area immediately adjacent to either side of the future weld site, and also by various minor changes to the weld cycle. The clamping operation excludes most of the fluid from the weld zone and makes it possible to join the tubes mechanically (but not necessarily sterily). This device is known as the DuPont SCD-II (U.S. Pat. No. 4,610,670).
The problem with the SCD-II is the flattening operation was "grafted on" to the existing SCD-I with only minor changes to the weld cycle. For some sizes of tubing SCD-II will produce sterile welds, but for other sizes the weld integrity will suffer.
The cause is in the flattening of the tubing prior to welding. During this process large stresses are built-up in the folding area of the tube. These stresses make it difficult to produce a reliable, sterile seal in that area. In addition, the motion of the tubes and their orientation during the re-alignment operation can cause viable spores to be drawn into the lumen area of the weld site.